1. Field of the Invention
The present invention relates to a charged particle beam exposure method and a charged particle beam exposure apparatus therefor, and in particular to a method for producing exposure data used for employing a charged particle beam, such as an electron beam, to expose a semiconductor wafer in order to form a pattern thereon, and to an apparatus therefor.
2. Related Arts
Sub-micron patterns can be formed by the irradiation of charged particle beams, such as electron beams, and such beams are employed in the manufacturing process for highly integrated LSIs. In particular, not only the charged particle beams are employed for the manufacture of masks, but the beams are also used to directly irradiate, and thus expose, resists formed on semiconductor wafers.
In the LSI design process, data for a pattern having a plurality of layers are produced to form a desired integrated circuit. In accordance with such pattern data, exposure of a resist on a semiconductor wafer or on a mask substrate is performed. During the exposure process, a resist film is irradiated by a charged particle beam in order to use the energy of the beam to induce a chemical reaction on the surface of the resist.
In this case, consideration must be given to the proximity exposure effect, which is due to the forward beam scattering and the rearward beam scattering which occur when a resist is irradiated by a charged particle beam. The proximity exposure effect is a phenomenon whereof when a charged particle beam is employed to irradiate a specific area, beam scattering causes the energy of the beam to spread to adjacent areas. For example, in a region having a high exposure pattern density, a developed pattern is expanded due to the energy of a charged particle beam which is projected onto areas adjacent to an exposure pattern. In a region having a low exposure pattern density, adjacent areas are not affected by the spread of energy, and a developed pattern is smaller or narrower.
Therefore, while taking the proximity exposure effect into account, designed pattern data must be corrected to obtain exposure data. The present applicant proposed a method for the preparation of such exposure data in Japanese Patent Application No. Hei 8-13354 (Japanese Unexamined Patent Publication No. Hei 8-321462), filed on Jan. 29, 1996.
According to the method proposed in this Japanese Patent Application, first, a quantity of exposure is set in accordance with the shape of a pattern, while taking the proximity exposure effect into account, and a plurality of mask areas are generated in a sub-field. The pattern densities in these areas are corrected, in accordance with the effects imposed by the pattern densities in the surrounding areas, to acquire substantial pattern densities while taking into consideration the proximity exposure effect. In accordance with the substantial pattern densities, the quantity of exposure (the strength of the exposure beam) of the pattern in each area is reduced, and in addition, for that area an auxiliary exposure patterns are produced.
According to the above method, however, the size of an area to be produced must be small, since pattern sizes are being reduced as a consequence of the recent highly integrated circuit trend. The number of areas is thus increased, and an extended period of time is required for the processing to prepare exposure data.
In addition, if the areas are uniformly created without consideration being given to the shapes and the locations of the patterns, a problem arises in that the corrected exposure data differ, depending on the individual areas for which they are generated, even if all the patterns are the same. In order to eliminate such dependency on the areas, one idea that has been proposed calls for the performance of a process for rearranging areas in accordance with the shapes and the locations of patterns. However, since in this case an additional associated data process must be performed, this idea is not advantageous.
Furthermore, as the number of areas is increased, there is a concomitant extension of the period of time required for data processing, in which the substantial pattern density is acquired from the pattern density of the area to review the quantity of exposure and to generate an auxiliary exposure pattern. As a result, the overall data processing throughput is reduced.
In addition, the generation of auxiliary exposure patterns is uniformly performed for a variety of exposure patterns in accordance with the pattern density of an area and the substantial pattern density resulting from an effect produced by the surrounding areas. However, in a special case, the quantity of exposure for an auxiliary exposure pattern may not be enough, and as a result, a narrow pattern may become much narrower.
In addition, according to the conventional method, even for an isolated pattern which does not require an auxiliary exposure pattern, an auxiliary pattern is generated if the pattern density in an area is low, with the result that there is an increase in the amount of exposure data prepared.
A beam energy expansion characteristic in a resist is not the same for all the resists. So unless auxiliary exposure patterns are formed while taking into account the beam energy expansion characteristic for individual resists, resist patterns having accurate pattern widths can not be formed. And when a plurality of auxiliary exposure patterns having different quantity of exposure are generated, this is accompanied by an increase in the amount of data and a reduction in data processing throughput.
Further, since the quantity of exposure established for a block mask is the same for all the patterns in the block mask, the quantity of exposure is normally set in accordance with the pattern having the minimum line width. This method, however, does not ensure that the optimal quantity of exposure will be set in all cases.